As a way of improving the volumetric and gravimetric capacity density of secondary lithium-ion batteries, production methods in which lithium in metallic form is accumulated within a negative electrode have been investigated as an alternative to the traditional methods that employ the intercalation of lithium ions into graphite. With the alternative methods, however, the lithium and the battery's organic electrolyte react, which when the battery charges/discharges turns the lithium into branchy crystals and gives rise to dendritic lithium growth that precipitates out of the electrode. As a consequence, the electrode performs less efficiently and the battery's cycling life is shortened. What is more, such lithium dendrites give rise to battery-internal shorting with the positive electrode, which presents a hazard that ultimately ends in the battery exploding.
One technique that has been investigated to date for curbing dendritic growth is the formation on the surface of the metallic lithium of a polymer film, or a solid electrolytic film such as a fluoride film, a carbonic film, an oxide film, an oxide-nitride film or a sulfide film; such films are disclosed in U.S. Pat. No. 5,314,765 (cf. claim 1), U.S. Pat. No. 6,025,094 (cf. claims 1 and 4), Japanese Unexamined Pat. App. Pub. No. 2000-340257 (cf. claims 6 and 7), and Japanese Unexamined Pat. App. Pub. No. 2002-329524 (cf. claims 1 and 9).
Given the objective of raising a lithium secondary battery's capacity per unit volume and weight, the metallic lithium must have a layer thickness that is kept to 20 μm or less, preferably to 5 μm or so, but freestanding lithium foil in that thickness range is so weak as to be unusable, thus rendering it necessary to use as a substrate a current-collecting material having strength, such as copper foil, and laminate the lithium foil onto it, or to form the lithium metallic layer onto a substrate by a gas-phase deposition technique such as chemical vapor deposition.
To date, an electroconductive substance such as copper foil has been employed as a negative-electrode substrate in secondary lithium-ion batteries.
Meanwhile, with techniques that form a solid electrolytic film onto metallic lithium to restrain the lithium from growing dendritically, in the course of producing and in the process of handling the negative electrode there is a likelihood that partial breakdown of the strongly hydrolytic metallic lithium layer and the sulfide-based solid electrolytic film will occur, which is assumed to be the coating effectiveness from the solid electrolytic film not manifesting itself. When such a situation occurs, in the compromised portions of the electrode the solid electrolytic film is destroyed and dendritic growth arises, which brings on a decline in the battery's cycling life. And in implementations in which an electroconductive material is employed as a substrate, the fact that electrons keep on being supplied to the negative electrode increases the likelihood that charging/discharging will concentrate where the electrons are being supplied. What is more, the encroachment of negative-electrode dendritic growth brings about battery-internal shorting between the negative and positive electrodes, which presents a hazard that ultimately ends in the battery exploding.